(1) Field of the Invention
The present invention relates to a process for fabricating metal filled contact holes, used in the manufacturing of semiconductor devices.
(2) Description of Prior Art
The semiconductor industry is continually striving to reduce the cost of manufacturing silicon chips, while still attempting to increase the performance of these same devices. The ability to reduce the dimensions of critical silicon device images or shapes, have allowed these objectives to be realized. The smaller shapes, accomplished via advances in specific semiconductor fabrication disciplines such as photolithography and reactive ion etching, have allowed smaller chips to be realized. This has resulted in more chips being produced from a specific size silicon wafer, thus reducing the cost of the chip. The smaller chip size, with smaller device features, also results in performance enhancements, resulting from decreases in capacitances and resistances.
However with the benefits of performance and cost, resulting from micro-miniaturazation, come specific reliability concerns. For example, smaller contact holes, filled with an aluminum based metallization, and used to connect active silicon regions to overlying interconnect metallizations, can result in electromigration problems. The decreased cross-sectional area of the aluminum fill results in higher current densities then counterparts fabricated using wider diameter contact holes, and may not survive the expected lifetime of the device, at used conditions. To overcome this risk the semiconductor industry has directed attention to metal fills exhibiting greater electromigration resistance.
Refractory metals, such as tungsten, are now being used to fill small diameter contact holes. The ability of tungsten to withstand very high current densities, allow the use of this metal for contact holes with diameters of less then 0.5 micrometers. However the use of tungsten requires the use of additional layers, used to line the contact hole, prior to the deposition of tungsten. The adhesion of pure tungsten, to the sides of the silicon oxide contact hole, would be unsatisfactory, thus an adhesion layer of titanium is usually used prior to filling the contact hole with tungsten. In addition, when the tungsten fill is accomplished via chemical vapor deposition, using a tungsten hexafluoride source, reaction product, such as fluorine ions, can result in the attack, or etching of the titanium adhesive layer. Therefore a barrier layer, usually titanium nitride, is used on the adhesive titanium layer, to protect the titanium layer during tungsten deposition. Therefore the industry has been using a composite layer of titanium--titanium nitride, to line the small diameter contact holes prior to tungsten fill.
Chen, et al, in U.S. Pat. No. 5,286,675, have described a composite titanium--titanium nitride layer, in conjunction with tungsten fills. However that invention, as well as others, does not address the difficulty encountered when attempting to uniformly coat all surfaces of the titanium adhesive layer with titanium nitride. As the aspect ratio of the contact hole increases, as experienced with the smaller diameter contact holes used with advanced designs, the conformality or step coverage of sputtered titanium nitride significantly decreases. If adequate barrier coverage is not achieved, and exposed regions of titanium are present, the subsequent fluorine attack, during the tungsten deposition, will result in a deleterious tungsten peeling phenomena, or adhesion loss. The invention now described will present a process in which robust titanium nitride barrier layers are achieved, protecting the underlying adhesive !aver of titanium from attack during the tungsten deposition.